The present invention relates to the manufacture of an anode electrode mix for electrochemical cells, i.e., batteries, and more particularly to a degassing process for densifying an anode mix for use in batteries.
Alkaline electrochemical cells commonly employ a positive electrode, referred to as the cathode, and a negative electrode, referred to as the anode. The anode and cathode are disposed in a container and are separated by a separator. The anode, cathode, and separator simultaneously contact an alkaline electrolyte solution. The conventional anode is commonly formed as a gel-type slurry of anode mix typically comprising zinc powder as the active material. In addition, the anode mix typically includes potassium hydroxide (KOH), binder (gelling agent), and additives.
The anode is typically manufactured in bulk at an anode manufacturing station to produce a large batch, often referred to as an anode slurry. The anode ingredients, including the zinc powder, potassium hydroxide, gelling agent binder, and additives, are uniformly blended together in a blender under atmospheric pressure conditions to form the anode slurry. In the conventional battery assembly process, the blended anode slurry is transported to the battery manufacturing assembly line where a fixed amount by weight of anode is dispensed into each battery container.
During the anode blending process, it is generally common for air to become entrapped within the viscous anode slurry. In addition, when the zinc powder comes into contact with the potassium hydroxide solution, hydrogen gas is created due to zinc corrosion reaction, also referred to as zinc gassing. The hydrogen gas created during zinc gassing is also entrapped within the viscous anode slurry. The amount of hydrogen gas generated by zinc gassing is initially greater during the initial contact of zinc with potassium hydroxide, and then the amount of additional hydrogen gas generated by zinc gassing stabilizes and dissipates over time. The presence of entrapped air and hydrogen gas in the anode slurry generally reduces the conductivity of the anode. The entrapped gases also reduce the density of the anode which can cause processing problems, and limits the amount of active electrochemical materials that can be placed in the cell. The commercially available anode slurries produced in conventional manufacturing facilities typically have a density that is in the range of about 94 to 96 percent of the theoretical density of the anode.
One approach to minimizing the presence of hydrogen gas in the anode slurry is disclosed in U.S. Pat. No. 5,632,554, which employs a flexible bag for mixing the anode components to prevent contact of the anode components with a metallic cylindrical mixing drum, to thereby avoid added formation of entrapped hydrogen gas bubbles in the slurry. An approach to removing entrapped hydrogen gas bubbles from an anode slurry is disclosed in U.S. Pat. No. 5,632,603, which employs a deareation chamber made up of a vacuum pumping apparatus having a vertical conduit, a downwardly inclined conduit, and an upwardly extending conduit. The vacuum pumping apparatus is subjected to a vacuum such that a flow of slurry is caused to move through the vertical conduit while the vacuum pump causes entrapped gas bubbles within the slurry to be subjected to a reduced pressure and to expand and burst, thereby releasing the gas from the slurry through the upwardly extending conduit. This approach enables the continual transfer and densification of the slurry while simultaneously eliminating gas bubbles. The aforementioned prior art approach requires slurry flow through conduits and is intended to provide continuous flow of anode from anode manufacture to cell assembly. Prior anode degassing approaches may not remove a substantial amount of the air and hydrogen gas that may be generated due to zinc gassing. As a consequence, a substantial amount of entrapped air and hydrogen gas may remain entrapped in the anode slurry.
Accordingly, it is therefore desirable to remove a significant amount of the entrapped air and hydrogen gas created during zinc gassing from the anode mix for use in electrochemical cells. It is further desirable to obtain an increased density of the anode mix to enhance the service performance of electrochemical cells.
The present invention removes entrapped air and hydrogen gas from an anode mix, and thereby increases the density of the anode mix for use in batteries. To achieve this and other advantages, and in accordance with the purpose of the invention as embodied and described. herein, one aspect of the present invention provides for an anode degassing process for removing gas and densifying a slurry of anode mix. The anode degassing process comprises the steps of providing an anode mix, aging the anode mix for at least thirty minutes following manufacture of the anode mix, disposing the aged anode mix into a mixer, generating a vacuum in the mixer, and agitating the anode mix by mixing the anode mix in the presence of the vacuum to remove gas from the anode mix, thereby providing an increased density anode slurry.
According to another aspect of the present invention, an anode slurry is provided for use as an electrode in an alkaline electrochemical cell. The anode slurry comprises zinc, an alkaline electrolyte such as potassium hydroxide, and a binder. The anode slurry is aged for at least thirty minutes following manufacture and agitated in the presence of a vacuum to remove gas from the anode slurry to produce an anode slurry having a density of greater than 96 percent of the theoretical density of the anode.
According to yet a further aspect of the present invention, an anode degassing apparatus is provided for degassing a batch of anode mix to remove gas and thereby increase the anode density. The anode degassing apparatus comprises a container, and a vacuum pump for creating a vacuum within the container. Anode mix is aged for at least thirty minutes, and is then agitated, such as by a mixer, in the container in the presence of a vacuum to remove gas from the anode mix.